template: Validate

template/template.go

 package template
 
 import (
+	"errors"
 	"fmt"
 	"time"
+
+	"github.com/hashicorp/go-multierror"
 )
 
 // Template represents the parsed template that is used to configure
@@ -68,6 +71,38 @@ type OnlyExcept struct {
 	Except []string
 }
 
+//-------------------------------------------------------------------
+// Functions
+//-------------------------------------------------------------------
+
+// Validate does some basic validation of the template on top of the
+// validation that occurs while parsing. If possible, we try to defer
+// validation to here. The validation errors that occur during parsing
+// are the minimal necessary to make sure parsing builds a reasonable
+// Template structure.
+func (t *Template) Validate() error {
+	var err error
+
+	// At least one builder must be defined
+	if len(t.Builders) == 0 {
+		err = multierror.Append(err, errors.New(
+			"at least one builder must be defined"))
+	}
+
+	// Verify that the provisioner overrides target builders that exist
+	for i, p := range t.Provisioners {
+		for name, _ := range p.Override {
+			if _, ok := t.Builders[name]; !ok {
+				err = multierror.Append(err, fmt.Errorf(
+					"provisioner %d: override '%s' doesn't exist",
+					i+1, name))
+			}
+		}
+	}
+
+	return err
+}
+
 //-------------------------------------------------------------------
 // GoStringer
 //-------------------------------------------------------------------

template/template_test.go

 package template
 
 import (
+	"os"
 	"path/filepath"
+	"testing"
 )
 
 const FixturesDir = "./test-fixtures"
@@ -10,3 +12,43 @@ const FixturesDir = "./test-fixtures"
 func fixtureDir(n string) string {
 	return filepath.Join(FixturesDir, n)
 }
+
+func TestTemplateValidate(t *testing.T) {
+	cases := []struct {
+		File string
+		Err  bool
+	}{
+		{
+			"validate-no-builders.json",
+			true,
+		},
+
+		{
+			"validate-bad-override.json",
+			true,
+		},
+
+		{
+			"validate-good-override.json",
+			false,
+		},
+	}
+
+	for _, tc := range cases {
+		f, err := os.Open(fixtureDir(tc.File))
+		if err != nil {
+			t.Fatalf("err: %s", err)
+		}
+
+		tpl, err := Parse(f)
+		f.Close()
+		if err != nil {
+			t.Fatalf("err: %s\n\n%s", tc.File, err)
+		}
+
+		err = tpl.Validate()
+		if (err != nil) != tc.Err {
+			t.Fatalf("err: %s\n\n%s", tc.File, err)
+		}
+	}
+}

template/test-fixtures/validate-bad-override.json

+{
+    "builders": [{
+        "type": "foo"
+    }],
+
+    "provisioners": [{
+        "type": "bar",
+        "override": {
+            "bar": {}
+        }
+    }]
+}

template/test-fixtures/validate-good-override.json

+{
+    "builders": [{
+        "type": "foo"
+    }],
+
+    "provisioners": [{
+        "type": "bar",
+        "override": {
+            "foo": {}
+        }
+    }]
+}

template/test-fixtures/validate-no-builders.json

+{}